1. Technical Field
The present invention relates to a pressure responsive device such as an electret condenser microphone or a pressure sensor for use in cellular phone or the like.
2. Background Art
FIG. 6 is a sectional view showing a conventional electret condenser microphone for use in cellular phone or the like. In the drawing, reference numeral 20 is a printed board on which a junction FET (hereinafter referred to as J-FET) 21 is mounted, and numeral 22 is a back plate. Numeral 23 is an electret membrane semi-permanently charged with an electrical charge (Q) by irradiating a polymer, e. g., polypropylene with an electronic beam. Numeral 24 is a spacer made of a plastic, and numeral 25 is a vibrating membrane disposed above the electret membrane 23 via the spacer 24 and coated with a surface electrode made of aluminum. This vibrating membrane 25 is opposite to the electret membrane 23 and the back plate 22 therebelow via a space, and forms a capacitor between these electret membrane 23 and back plate 22 and the vibrating membrane 25. Furthermore, numeral 26 is a retaining rubber for fixing the vibrating membrane 25. Numeral 27 is a holder for holding the back plate 22 and the electret membrane 23. Numeral 28 is a capsule including a vent hole 29, and numeral 30 is a cloth covering the vent hole 29.
In the conventional electret condenser microphone, the capacitor is constructed of the back plate 22, the electret membrane 23 and the vibrating membrane 25 having the surface electrode. When a sound pressure such as a sound or voice is transferred through the vent hole 29 of the capsule 28, the vibrating membrane 25 is vibrated by this sound pressure thereby a capacity (c) of the capacitor being varied. Since an electrical charge (Q) is constant, variation in a voltage (V) is produced on the basis of Q=CV. Applying the voltage variation to a gate electrode of J-FET 21 causes variation in drain current, which is detected in the form of voltage signal.
Since an electret condenser microphone is used for a take-along terminal, e. g., a cellular phone, further thinning and miniaturization thereof have been desired. In the conventional construction of above construction, however, the printed board 20, J-FET 21, the holder 27 and the like are used resulting in a large number of parts. Therefore thinning and miniaturization of the electret condenser microphone were difficult. Moreover in the mentioned conventional construction, a problem exists in that S/N ratio is lowered as being thin and small-sized, eventually resulting in worse performance.
The present invention was made in order to solve the above-discussed problems, and has an object of providing a pressure responsive device capable of achieving thinning or miniaturization thereof while maintaining a high performance. The invention also provides a method of manufacturing a semiconductor substrate for use therein.
A pressure responsive device according to the invention comprises:
a package including a storage chamber in an interior thereof; means for introducing an outside pressure into the storage chamber;
a semiconductor substrate placed in the storage chamber; and
a capacitor placed on the semiconductor substrate and of which capacity varies according to the outside pressure introduced into the storage chamber;
wherein a concave having a bottom surface and a peripheral surface surrounding the concave are formed on one main surface of the semiconductor substrate, the capacitor is provided with a fixed electrode membrane placed on the bottom surface of the concave and a vibrating electrode membrane fixed on the peripheral surface so as to cover the concave and facing to the fixed electrode membrane through a space, and the vibrating electrode membrane vibrates according to variation in the outside pressure introduced into the storage chamber.
In the pressure responsive device according to the invention, it is preferable that the peripheral surface is a flat face positioned on a first plane, and the bottom surface of the concave has a flat face positioned on a second plane spaced away from and substantially parallel with the first plane.
In the pressure responsive device according to the invention, it is preferable that the semiconductor substrate includes a conversion circuit for converting variation in capacity of the capacitor due to vibration in the vibrating electrode membrane into a voltage signal.
In the pressure responsive device according to the invention, it is preferable that the semiconductor substrate is provided with communication means for communicating the space and the storage chamber.
In the pressure responsive device according to the invention, it is preferable that the communication means includes a communication groove running from the concave to an outer edge of the semiconductor substrate is formed on the one main surface of the semiconductor substrate.
In the pressure responsive device according to the invention, it is preferable that the semiconductor substrate has another main surface opposite to the mentioned one main surface and has an air vent hole running from the concave to this another main surface.
In the pressure responsive device according to the invention, it is preferable the package has an air vent hole on a bottom wall that overlaps with the air vent hole of the semiconductor substrate.
In the pressure responsive device according to the invention, it is preferable that the concave is in the range of 5 to 15 xcexcm in depth.
In the pressure responsive device according to the invention, it is preferable the vibrating electrode membrane includes an electret membrane made of a polymer which is electrically charged and an electrode formed on the electret membrane.
In the pressure responsive device of above construction according to the invention, a fixed electrode membrane is placed on the bottom surface of the concave formed on the one main surface of the semiconductor substrate and the peripheral edge portion of the vibrating electrode membrane is fixed on the peripheral surface of the semiconductor substrate surrounding this concave, thereby forming a capacitor comprised of the fixed electrode membrane/the space/the vibrating electrode membrane. As a result, according to the invention, number of parts becomes smaller than that in the conventional apparatus of same type and moreover each part is thin and small-sized, and consequently it is possible to achieve thinning and miniaturization of the apparatus while maintaining a high performance.
In the mentioned pressure responsive device in which the peripheral surface of the semiconductor is the flat face positioned on the first plane, and the bottom surface of the concave is a flat face positioned on the second plane spaced away from and substantially parallel with the first plane, it is possible to obtain sufficiently large variation in capacity value of the capacitor according to variation in outside pressure.
In the pressure responsive device in which the semiconductor substrate is provided with the conversion circuit for converting variation incapacity of the capacitor into a voltage signal, any special part serving as a detecting circuit is not required and it is possible to obtain a smaller-sized pressure responsive device.
In the pressure responsive device in which the semiconductor substrate is provided with communication means for communicating the space and the storage chamber, air in the space easily gets in and out the storage chamber, and it is possible to easily vibrate the vibrating electrode membrane.
In the pressure responsive device in which the communication groove running from the concave to the outer edge of the semiconductor substrate on the one main surface of the semiconductor substrate, it is possible to easily form the communication means on the semiconductor substrate.
In the pressure responsive device in which an air vent hole running from the concave of the semiconductor substrate to another main surface is formed, air in the space easily gets in or out and, and it is possible to easily vibrate the vibrating electrode membrane.
In the pressure responsive device in which the package is also provided with an air vent hole communicating to the air vent hole of the semiconductor substrate, it is possible to give a substantially constant pressure from outside of the package to the space and effectively vibrate the vibrating electrode membrane.
In the pressure responsive device in which the concave is in the range of 5 to 15 xcexcm in depth, it is possible to reduce influence of variation in depth of the concave and assure a moderate sensitivity.
In the pressure responsive device in which the vibrating electrode membrane includes the electret membrane made of a polymer which is electrically charged and the electrode formed on the electret membranethe, it is possible to effectively obtain variation in capacity value of the capacitor due to vibration of the vibrating electrode membrane.
A method of manufacturing a semiconductor substrate used in a pressure responsive device according to the invention, the semiconductor substrate having a concave with a bottom surface, a peripheral surface surrounding the concave, and at least one communication groove running from an inner circumference to an outer circumference of the peripheral surface on one main surface,
the method comprising:
a first step of forming a first resist membrane on the entire one main surface of the semiconductor substrate;
a second step of patterning the first resist membrane so as to form an opening while leaving the first resist membrane on the peripheral surface, the opening is positioned on an inner portion of the peripheral surface;
a third step of forming a concave of 5 to 15 xcexcm in depth through the opening using the first resist membrane as a mask;
a fourth step of removing the first resist; a fifth step of forming a second resist membrane so as to cover the concave and the peripheral surface;
a sixth step of patterning the second resist membrane so as to expose at least one passage running from the inner circumference to the outer circumference of the peripheral surface; and
a seventh step of forming a communication groove of 2 to 3.5 xcexcm in depth on the passage using the second resist membrane as a mask.
In the method of manufacturing a semiconductor substrate according to the invention, it is possible to form a concave on the one main surface of the semiconductor substrate through etching, and it is therefore possible to reduce variation in depth of the concave in device. As a result, it is possible to reduce variation in performance of each device and to produce highly reliable pressure responsive device in large quantities at a reasonable cost.